The present invention relates generally to the field of telecommunications and, in particular, to an intermediate frequency video encoder used in conjunction with telecommunication transmission systems.
Currently, one type of system for the transmission of a signal includes the use of fiberoptics due to the high speed and long distance transmission capability of such a system. For one such system, signals (e.g., a video signal) having different data rates are modulated onto an intermediate frequency (IF) carrier and then digitized and formatted to a fixed data rate and transmitted over digital fiberoptic transmission lines.
Different encoders have been designed to digitize and format fixed data rate signals for transmission over the fiberoptic transmission lines. The design of such encoders involve the downconverting and digitizing of a bandpass input signal. Currently one approach in the design of such encoders has included analog mixing of the input signal prior to the analog to digital (A/D) conversion. Disadvantageously, this approach requires a relatively large amount of analog circuitry to precede the A/D conversion. Accordingly, any modification in the input signal, the sampling rate and/or input signal passbands of the encoder, or other design modifications translates into a significant amount of redesign of the analog circuitry. In other words, this type of approach is not very extensible due to the requirement of a redesign of the analog circuitry when there are modifications in the input signal or design of such an encoder.
A second approach to the design of such encoders has relied on undersampling followed by a fixed rate decimation of the A/D output data signal. The decimation process reduces the data rate of this A/D output data signal; however, this reduction is such that the input/output ratio of the data signals of the encoder must be integer-based. This approach has traditionally been used for the downconversion and digitization of narrowband (i.e., less than 500 kiloHertz (kHz)) bandpass signals. Disadvantageously, because of the requirement that the input/output ratio be integer-based, this approach is limited to working for certain discrete passbands.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an intermediate frequency encoder that has a higher performance and minimal analog circuitry as well as one that outputs a signal at a fixed data rate for input signals having a wide range of data rates and frequency spectrums.
The above mentioned problems with communication systems and other problems are addressed by the present invention and which will be understood by reading and studying the following specification. An encoder and a method for encoding that can be used in conjunction with communications systems is described. In particular, embodiments of such an encoder can be incorporated into fiberoptic communication systems wherein the encoder digitizes and formats an analog signal for transmission across fiberoptic transmission lines. The input signals to such systems are analog signals having both narrowband (i.e., less than 500 kHz) and wideband (greater than 500 kHz) bandpass signals. Further, embodiments of the encoder allow for extensibility as a relatively minimum amount of analog circuitry is incorporated in the design of such encoders. Accordingly, the majority of the signal processing and formatting of the signal is performed digitally.
In particular, an illustrative embodiment of the present invention includes an encoder. The encoder includes an analog-to-digital (A/D) converter that receives an analog signal. The A/D converter lowers the frequency spectrum of the analog signal. Additionally, the A/D converter digitizes the analog signal. The encoder also includes a resampler that is coupled to the A/D converter. The resampler receives the digital signal and converts the digital signal to an output signal. Further, the data rate of the output signal is independent of the data rate of the sampled analog signal.